Powerful plants have changed the world

May 10, 1999: Biologists conducting Space Shuttle
experiments may be one step close

r to shedding light on the biggest power
booster on the planet: a protein in green plants called Photosystem
I.

A German research team recently presented the results of their
Space Shuttle experiment designed to crystallize Photosystem
I molecules. According to the researchers, "This experiment
has yielded the best data set thus far obtained from Photosystem
I crystals."

Right: Photo credit: Department of Energy, Daniel Peck

During

photosynthesis, the cells in green plants undergo
two simultaneous reactions, both of which rely on a separate
kind of protein. Photosystem I protein molecules use the trapped
energy in sunlight to convert carbon dioxide into carbon and
oxygen. This provides the plant food in the form of carbohydrates,
lipids, proteins and nucleic acids - the building blocks of life.
Photosystem II protein molecules use light energy to split water
into hydrogen and oxygen for plant respiration.

Scientists crystallize protein molecules in order to study
their complex internal structures. Because the molecules are
too small to study directly under a microscope, scientists use
X-ray diffraction to get a picture of the molecule.

Shining X-rays through a crystal produces a scattering pattern,
which is a type of blueprint. Think of a shadow cast through
a picket fence - the shape of the shadow would tell you that
the fundamental building block of the fence is a rectangular
board. Shining X-rays through a protein crystal indicates the
protein's shape, where it's located, and ultimately how it may
work.

Left: In the microgravity environment of the
Space Shuttle, scientists have shown some improved capability to
grow macromolecular crystals with a higher degree of order. Using
a process called "X-ray crystallography," they can
map the structure of proteins and advance the fundamental understanding
of how they work. more
information

High quality crystals - composed of ordered and repeating
units of a particular protein - are required for X-ray diffraction.
Some of the crystals grown in the microgravity conditions of
space are more perfectly ordered than crystals grown on Earth.
Microgravity can also affect the rate at which the proteins initiate
new growth. Space crystals have shown a 10 to 20-fold larger
volume compared to the Earth-grown counterparts.

The
Photosystem I protein molecule, sometimes called "the Earth's
power station," was analyzed by a scientific team representing
the Max Volmer Institute for Biophysical Chemistry and Biochemistry
in Berlin, Germany. The team reported their results in their
Final Report published from the Life and Microgravity Spacelab
(LMS) mission. The team hopes these results will give scientists
a more detailed knowledge of the Photosystem I molecule's shape,
exact atomic positions, and biological functions. And by using
the results of the experiments on the space shuttle, scientists
can improve the crystallization conditions here on Earth.

Right: Green plants use Photosystem proteins to capture
and use energy from sunlight. Photo credit: Department of Energy,
Daniel Peck.
The Earth's environments - from forests to

grasslands to the oceans - are direct
products of the Photosystem protein molecules. From the beginning
of life, Photosystem processes in algae completely altered the
atmosphere, transforming the carbon dioxide environment into
an oxygen-rich one.

Left: Algae in the early Earth's oceans transformed
the atmosphere. Photo credit: Department of Energy, David Parsons.

The two Photosystem proteins underlie the Earth's balance
between water and heat and between oxygen and carbon dioxide.
They ultimately supply the nutrients for almost every living
thing on the planet, as well. Most of the organisms on Earth
receive their sustenance directly or indirectly from photosynthetic
vegetation. Without the Photosystem molecules, life as we know
it would cease to exist.

The space experiments were performed on ancient
organisms called cyanobacteria, formerly known as blue-green
algae or blue-green bacteria. As a family, these organisms form
the fundamental basis of the entire marine food web and are often
called "the grass of the sea." These early ancestors
of modern plant cells (chloroplasts) were the first oxygenic
organisms to convert light to energy on Earth. The cyanobacterium
protein used in the space investigation, from the species Synechococcus
elongatus, is found abundantly today. It represents more
than half of the total biomass productivity in all open ocean
environments and may process up to 50 percent of the excess carbon
dioxide greenhouse gasses implicated in the current global warming
debate.

Burning carbon fuel such as oil and coal produces most of this
excess carbon dioxide. This process currently supplies much of
the world's power needs, but the fuel reserves are rapidly running
out. Nonpolluting alternative fuel sources are being developed
to take the place of oil and coal. In the 1970s, solar power
- a clean and unlimited power source - seemed to be the most
promising alternative. Harnessing the power of the Sun to power
the Earth, however, has been plagued with difficulties. To generate
a lot of power, you need extremely large solar panels. And what
do you do for power when the sun sets?

The Space Shuttle investigation is trying to discover
what features of photosynthetic proteins allow for solar energy
conversion. While humans have only been developing solar power
technology for a few decades, plants have been evolving for billions
of years to perfect their photosynthetic technique. By studying
how plants accomplish this remarkable feat, scientists hope to
someday also develop systems that use light as a power source.
Identifying and studying characteristics of the protein's metabolism
may someday also be used for applications in pollution prevention
and environmental clean-ups.
Knowing the CodeMany essential biology
questions depend on knowing the structure of proteins and enzymes.
By charting their shape, scientists can determine how the molecules
work. But these molecules may also change shape when performing
important functions, like carrying oxygen in blood hemoglobin.
In photosynthesis, there are many energy producing conversion
steps from sunlight to plant development and growth.

Some estimates suggest that human biology depends on the action
of nearly half a million different enzymes and proteins. But
we only have a three-dimensional picture of shape and function
for fewer than 1 in 100 of these complex chemicals. Since 1984,
the Space Shuttle has carried experiments to determine the structures
of large, biologically important molecules. This research has
compiled results for a host of human diseases ranging from insulin
for the control of diabetes, to the reverse transcriptase enzyme
that, when blocked, inhibits HIV infection.

Just as in human cells,

the Photosystem proteins inside a plant
cell are translated from amino acids. Amino acids have a 20 letter
alphabet for each of the 20 naturally occurring amino acids (shown
below as AAs). These amino acids are in turn translated from
the complex array of nucleic acids in DNA (coded as the letters
A,G,T and C). A description of the molecular code reads like
an encrypted message:

Much work remains to be done in uncovering the shape and detailed
way the Photosystem power-converting molecules achieve their
efficiency. By using the results from space shuttle experiments,
someday we may understand how that transformation happens in
detail. Such experiments make possible the study of proteins
that had once proved too difficult to dissect at the molecular
or atomic size.
Life on the Edge Project

A good illustration of how photosynthesis leads to
environmental balance is the terrarium. A sealed jar of carefully
balanced photosynthesizing organisms can sustain themselves for
long periods without exposure to outside material nutrients or
gases. Such a microbial terrarium can keep its ecological balance
nearly indefinitely without any care or maintenance.

The secret to this self-sufficiency is that green or purple photosynthesizing
organisms generate their own source of life from the energy in
light. This ability allows them to divide and multiply in a stable
manner. NASA's "Life on the Edge" project tests some
of the limits to this remarkable behavior. By closing several
green biomass mixtures into sealed jars, these organisms are
frozen within a deep freezer to a frigid -80 deg C (-112 deg
F), temperatures exceeding the coldest winter weather in Antarctica
(-44.5 deg C, or -48 deg F). Afterwards, the jars are thawed
and opened, and scientists then grow the organisms in a culture
to assess their viability. Healthy growing ecosystems have been
revived from this ultimate deep freeze.

This is one of several stories summarizing results
from the 16-day Life
and Microgravity Spacelab (LMS), which flew June 20-July
7, 1996, aboard Space Shuttle Columbia (STS-78, at launch, left).
It featured 40 scientific investigations from 10 countries. Its
record development and cost - each experiment cost about half
of most Spacelab experiments - make LMS an example of how future
space station missions can control experiments remotely from
locations around the globe. LMS results were recently published
by NASA (see below). The investigation in this story used the
European Space Agency's Advanced Protein Crystallization Facility.

Other LMS stories:

Nature's sugar high - Spacelab
successfully crystallizes an intensely sweet protein from the
African Serendipity Berry that has 3000 times the kick of table
sugar - and no calories.

Great Bugs of Fire - Spacelab
crystallizes a protein from a very weird, and surprisingly common,
volcano-loving bug. Scientists hope to discover how these organisms
can survive in such extreme conditions.

Nature's "electronic ink" - Another extremophile - a bacterium which thrives
in high-salt conditions - produces a fascinating protein which
changes color extremely efficiently. Crystals grown by Spacelab
make scientists hopeful that they can understand the biological
function and apply it to, for example, artificial retinas for
people.